Thanks to quarterly SEC filings, we learned Tuesday that Eddie Lampert, Chairman of Sears Holdings (SHLD) and General Partner of the hedge fund ESL Investments, has been buying shares of Citigroup (C) since early 2006. At the end of the first quarter Lampert had amassed more than 15 million shares worth about $800 million. The filings show that Lampert began buying Citigroup in the first quarter of 2006 at prices in the mid to high forties. Today shares are jumping 1.7 percent in the pre-market to more than $53 each.

The purchase makes sense given that Lampert is a value guy (Citi trades at a 10 P/E and yields 4%) and his hedge fund is big enough that large cap stocks are the only kinds of investments that he can really take a meaningful position in without buying an entire firm. I've seen various press accounts of the Citigroup purchase speculating that Lampert is planning on using his stake to put pressure on the company to make significant changes. However, those hoping for shareholder activism on ESL's part shouldn't get too excited. Although $800 million is a lot of money, Lampert now owns less than one half of one percent of Citigroup. Hardly enough to play the hedge fund activism card.

Full Disclosure: Long shares of Sears Holdings and no position in Citigroup at the time of writing